Projection screens



Nov. 13, 1962 A. l. MIHALAKIS ETAL PROJECTION SCREENS 2 Sheets-Sheet 1Original Filed Aug. 5, 1953 "Z w M N mwm W 1,. n Z M; i

Nov. 13, 1962 A. I. MlHALAKlS ETAL 3,063,339

PROJECTION SCREENS 2 Sheets-Sheet 2 Original Filed Aug. 5, 1953 PatentedNov. 13, 1962 3,063,339 PROIECTEQN fifiCREENS Agis I. Mihalalsis, CanogaParis, Calif, James L. Read, Eggertsville, N.Y., and John L. Curtin, St.Paul, Mum, assignors, by mesne assignments, to William J. Snyder, SouthLancaster, Mass.

Continuation of application Ser. No. 371,822, Aug. 3, 1953. Thisapplication June 18, 1958, Ser. No. 744,310 (Filed under Rule 47(a) and35 U.S.C. 116) 11 Claims. (Cl. 88-289) The present invention relates tooptical screens of the type suitable to receive an image formed byprojection apparatus and to present the image to observers. This is acontinuation of application Serial No. 371,822, filed August 3, 1953,now abandoned.

Copending application of Agis I. Mihalalcis, one of the inventorsherein, Serial No. 257,691, filed November 23, 1951, now Patent No.2,804,801 of September 3, 1957, and divisional application thereofSerial No. 683,437, filed Aug. 27, 1957, now Patent No. 2,984,152 of May16, 1961, discloses and claims a screen of the above type which hasparticularly favorable brilliance and field properties, some of itsprincipal novel features being a definitely controllable field ofobservation, uniform distribution of the light intensity relayed fromprojection apparatus to this field of observation, maximum efficiencybecause of the definite separation of the field of observation from theenvironmental field and because of minimum absorption of light energy atthe screen, and favorable rejection of light impinging on the screenfrom without the field of observation so that, at any point of the fieldof observation, such external light is effective essentially only atintensities which are at such points lower than those purposely directedthereto.

The present invention is a further development of the system accordingto the above copending application, and some of its objects aretoprovide a screen of this general type which permits, by means of aminimum number of basic optical patterns of maximum efiiciency,extensive adjustment and regulation of the field of illumination bycomparatively inexpensive and easily controlled and regulatedexpedients. Other objects are to provide improved screen structures ofmaximum optical efiiciency and mechanical strength.

In one of its principal aspects, the invention provides a system ofcontrolling and predetermining the field properties of a projectionscreen by the controlled formation from a master-pattern of anon-absorptive, optically smooth and continuously curved compositeimaging surface, which field control system is adapted to include, inaddition to the shaping of the optical imaging surface, additionalcontrol by way of predeterminable, essentially non-absorptive scatteringof beams carrying image patterns received and formed or relayed by thenon-absorptive imaging surface.

In another optical aspect of the invention, an optical screen comprisesreflective or refractive image forming elements of minimum absorptionand maximum continuity, which elements are continuously joined ataperture defining contours. These contours or apertures and the imagingpowers of the elements together define the vertical and lateral fieldangles of a field of projection or observation. This screen can be usedindependently or in combination with essentially transparent ortranslucent scattering elements which for the purpose of adjustment offield properties, are superimposed on or jutaposed to the image formingelements.

In an especially useful embodiment, the scattering elements are embeddedin a thin, transparent layer of essentially uniform thickness which isapplied to a metal mirror of maximum continuity and smoothness, andhence maximum reflection but minimum absorption of light energy.

These and other objects and aspects of the invention will appear, fromthe herein presented outline of its prin ciples, its mode of operationand its practical possibilities together with a description of severaltypical embodiments illustrating its novel characteristics. These referto drawings in which FIG. 1 is a schematical fragmentary section througha screen according to the invention;

FIGS. 2 to 5 are diagrams depicting the configuration of imagingsurfaces according to the invention;

FIG. 6 indicates the scatter elements on an imaging surface according tothe invention;

FIG. 7 is a diagram illustrating the image modifying effect of a scattercomponent according to FIG. 6;

FIG. 8 is a schematic section through a practical reflecting screenstructure according to the invention; and

FIG. 9 is a comparative diagram of screen performances.

Generally speaking, screens according to the present invention arecomposed, as indicated in FIG. 1, of an imaging sheet R and a scatteringlayer S. The imaging sheet has an optical surface s which can be part ofa refracting lens structure, or constitute or support a reflects TheImaging Component The optical function of this component depends mainlyon its field properties and on its resolution, as required by itsintended use.

The field properties are determined by the stops of the imagingelements. The entrance pupil of an imaging system is defined as theimage of the aperture stop in the object space while the exit pupil isthe image of the aperture stop in image space. Since the principalembodiment herein disclosed employs a reflector system we shall considera mirror in which the mirror margin is the aperture stop which then alsobecomes the entrance and exit pupil of the system.

The configuration of the field of observation depends on the use of thescreen. For example, if a screen is designed for use in an educationalaudio-visual program, a lateral viewing angle of approximately 90 ispreferred since the screen can then be positioned in a front corner ofthe room and cover all the students without any rearrangement of seats.The vertical viewing angle is selected with regard to the particularheight of the screen whose field must include all students when seated.Also, the vertical field should be restricted in size so far as possiblein order to conserve energy.

A screen according to invention, designed for such an observation field,is not perceptibly affected by illumination from the side windows orceiling lights.

As a basis for determining the resolution it is safe to assume that theeye can just resolve two objects separated by an angle of 70 seconds.Using this information about the configuration of a given field ofobservation, together with the fact that no person should sit closer tothe image than twice the width of the screen, the maximum elementarydiameter of each imaging element can be computed.

The element size and field configuration having been determined, theoptimum configurations of the screen elements can now be computed. Whilea certain choice is available in this respect, there is an optimumsurface which meets all the requirements of good design for a givenfield of observation. Theoretically, either convex or concave elementsof properly selected configuration can be used. A field of observationwith different horizontal and vertical field angles can be provided byappropriately dimensioning, either or both, diameter and curvature ofthe elements. However, for several reasons, all convex or all concaveelements do not provide optimum effectiveness. (Jonsidering themanufacturing aspects, it is practically almost impossible to providegood polish in the vicinity of the mirror stops. This leads to energyabsorbing areas and to areas of non-image forming refiection, which makesuch a screen undesirable for general use. Hots spots form mainly whereconcave or convex elements are adjacent with theoretically sharp edgeson the boundaries. Actually, flat surfaces at these regions can not beavoided so that a screen of such design operates at these regions like aplane mirror at least for certain viewing angles. Energy absorbing deadspaces appear at theoretically sharp concave edges, and wherever dirtcan accumulate. Both types of deficiency are practically unavoidable ifa reflecting surface is applied by processes which inherently can notavoid nonuniform metal distribution, such as spraying.

The above copending application provides a screen configuration withoutany of these defects. By Way of example, two specific screens of thistype will now be described which are found especially useful forpurposes of the present invention.

The first example is designed for a lateral angle of 107.2 and avertical angle of 82, and is that described in the above copendingapplication. The second example is designed for lateral and verticalangles of 100 and 50 respectively. Regarding resolving power and ienceelement size, both patterns are based on the dimen sions of acommercially available screen which is 40 inches wide, and on the abovementioned practice that the closest distance from the screen is,according to accepted practice, about twice its width.

The topographical map of FiGURE 2 illustrates the flow pattern of ascreen of this type. The solid lines marked A, B, C, D, of this figuredenote the sections or curvatures which occur when progressing in anydirection. The dotted lines H and V indicate the horizontal and verticalaperture stops respectively for the various elements, shown separatelyand similarly indicated in FIG. 3. it will be further noted that theelements alternate in curvature along lines such as A-DA, B--CD, so thata convex, concave pattern is maintained throughout. The curvatures atright angles to these lines such as those indicated at ABA, CDCdetermined the horizontal field and are all concave in nature while theadjacent row of elements in a horizontal direction are all convex. Thesecurvature lines are marked I, II, III, IV. As a result of thesecurvatures, the vertical field of observation is maintained byalternating convex and concave elements. The horizontal field iscomposed of vertical rows of concave elements adjacent on either side tovertical rows of convex elements.

FIGS. 4 and 5 indicate the various surface levels with reference to thelegends of FIGS. 2 and 3. The sagitta of the curves are selected toleave no part of the surface, so to speak, undesigned or having evenresidual hot spots or dead spaces of the above discussed detrimentalnature. The depth of a curve in one direction is adjusted to meet thedepth of the curve running at right angles to it and at the same timeretain the field of view desired. Thus the figure shows reference levelscorresponding to the elevation levels of the various elements.

By means of conventional method it is possible to compute the radii ofcurvatures, the elements diameter and 4 the sagittae giving an angle ofobservation in one direction which is compatible with the correspondingdata of a different an le of observation at right angles to the first,the two angles together determining the field of observation.

The specific dimensions for the two embodiments are given in the tablebelow which refer to the corresponding symbols of the various valuesindicated in FIGS. 4 and 5.

in this table, the dimensions marked I are those of the first mentionedscreen which is also described in the above mentioned application,whereas those marked II refer to an additional embodiment, alldimensions being in inches.

The above described imaging screen is in all essentials, although not asto now preferred dimensional details, similar to one dealt with inPatent No. 2,804,801 of Sept. 3, 1957.

The Scattering Component As mentioned above, the present inventioninvolves the combination of the above described imaging component with ascattering component which will now be described.

Generally speaking, the scattering component has the function ofcontrolled spreading the field of obsewation without appreciablyimpairing the uniformity of energy distribution within that field or theefliciency of energy transmission between proiector and observer. Itaffects the image formation in a manner which while it detracts from theoptical properties of the elementary images in the conventional sense,neverthelses improves these properties for purposes of the invention.The general construction of the combined imaging and diffusingcomponents will first be explained with reference to FIG. 6.

FIG. 6 indicates an elementary surface s, which may pertain to arefractive or reflective system. Assuming by way of example a reflectivesystem, s is a highly reflective metal surface supported on a basestructure which will be described in detail below. The scattering effectaccording to the invention can be accomplished by way of a separatestructural component, or by means of a special surface treatment. Ineither case, a coating D is applied to the reflecting surface s. Thiscoating is preferably of uniform thickness and carries scatteringelements P of an order of magnitude slightly larger than the longestwave length of the spectral range to be handled, in most cases theentire visible range. Either superficial or imbedded elements can beused but an embodiment with imbedded scatterers will first be described.

FIG. 6 indicates scattering particles P imbcdded in the carrier coatingD. Both carrier D and particles P are dielectric and examples ofappropriate materials will be given hereinbelow. As well known,particles of this type which are somewhat greater than one wave lengthdisplay phenomena which result in a somewhat spread reflection of theincident light. However, the particle size is in the nagnitude of aboutten times the size of the scattering particles and its material shouldbe as transparent as possible. The size of the scattering particles Pshould be of the general order of magnitude of about a thousand to twothousand millimicrons, or generally speaking somewhat larger than thelongest wave length of the light to be handled. The outer surface of thelayer S should follow the outline of the reflecting surface s, in otherwords the two surfaces of the coating S should be parallel, with nofilling in of concave elements or diminishing of thickness at convexelements. The density and distribution of the diffusing particles oflayer S should be such that transparency is essentially preserved so asnot to impair the reflective effect of the surface s. About 5% by weightof scatter material to carrier material, of the types described below,was found to be satisfactory.

It will now be evident that the dimensions of the scattering elementsare of an order of magnitude which is essentially lower than that of theimage forming elements.

In this manner, the predetermined field properties of purely imagingreflecting or refracting non-diffusing screen of the type described withreference to FIGS. 2 to 5 can be changed at will by adjusting thescattering structure incorporated in layer S, although the exact changecannot always be theoretically predicted and depends a good deal onlaboratory experimentation which, however, is fairly simple and does notappreciably add to the cost of the screen and the reliability of thepredicted performance. However, the practical performance agreesgenerally speaking with the theory of this device, as herein indicated.Practically uniform energy distribution over the field of observation,with an energy distribution (screen brightness) inversely proportionateto the field area is obtained, which indicates that the scatteringcomponent absorbs very little energy and thus does not detract from thehighly efficient performance of the imaging component with itspractically non-absorbtive, uninterrupted surface.

Combined Efiect FIG. 7 indicates the combined effect of imaging andscattering components. The field widening effect of the scatteringelements is probably due to the fact that the scattering pattern ofpencils at points near the aperture margin of the reflecting elementswill be unsymmetrical, whereas it becomes more and more symmetricaltowards the center and is essentially symmetrical with regard to ray a.Consequently, an image spread results, as indicated in FIG. 7 where Omand On are the scatter images for the marginal pencil ml, whereas 0 isthe corresponding unscattered image.

Whatever the theoretical basis may be, the scattering component does notaffect the uniformity of brilliance within a certain field, although itproduces a zone of more or less diffused light around the theoreticalfield for the undiifused reflector. For example, for a 40 lateral fieldfrom the normal of the uncoated reflector, the diffusion layer willprovide unimpaired brilliance over a field from the normal of about withfairly uniform intensity distribution, but the intensity falls graduallyoff to eventually zero at 90 off the normal, with a total field of about80 very well usable for average purposes. This effect is illustrated inFIG. 9, wherein curves XI and XII illustrate the effect of scattercoatings to be described in detail hereinbelow.

Instead of imbedding scattering particles within a coating, a similareffect can be obtained by applying to an image forming surface apractically transparent layer whose outer surface is modified to providescattering but essentially non-absorbing irregularities. It was foundpossible to apply such a coat which has slight surface irregularities ofa configuration, size and distribution which produces the above outlinedscattering effect without detriment to the primary imaging function ofthe elementary mirrors or lenses. Similar to the scattering particles,these surface irregularities can be controlled to provide predeterminedtheoretically predicted and experimentally selected variations of thefield intensity and configuration properties of purely imaging screens.

Practical embodiments of the above discussed concepts will now bedescribed.

Practical Embodiments The outer layer S is in a practical embodiment, aroll coating consisting of one part thinning medium consisting of equalparts of toluol and isopropylacetate added to four parts of an acryliccompound made up of nitrocellulose and the copolyrner known as butylmethacrylate. The proportion is approximately 35 parts by weight of theresinous materials to 4 parts of nitrocellulose, sufficiently nitratedand of low viscosity for lacquer formation. To each five gallons of thismixture are added approximately 23 ounces of the material availableunder the trade designation Aerosil for Godfrey L. Cabot Inc., ofBoston, Mass, which material consists essentially of colloidal silica ofa grain size of approximately .015 to .020 micron.

It appears that these colloidal particles occur in aggregates of anaverage size of about a thousand millimicrons, in keeping with the aboveoutlined scattering function thereof. This roll coating is approximately5 mils thick and is applied with a conventional surface roll-coatingmachine with a glue-line separation of any desired thickness, in aspecific embodiment about .003 inch.

The metal layer 2 can comprise any highly light-reflecting specularmetal, and in the embodiment of FIG. 8 it comprises a plating of highlyspecular metal such as silver or aluminum applied directly to a plasticsheet 3 in any known way. For example, the metal can be applied througha vacuum plating process wherein the plastic material is degassed to thenecessary degree and then exposed to a metal vapor to provide a thinmetallic coating of uniform thickness.

The plastic sheet 3 is a synthetic thermoplastic material which isnonreactive and inert with respect to the metallic layer 2 even whenheated to temperatures ap proaching its flow temperature. Otherwise,when the product is heated to molding temperatures there is a serioustendency for the plastic material to separate from the metallic layer,and for the metallic layer to deteriorate and completely lose its highlylight-reflective, non-absorptive characteristics. Thus, it is essentialthat the plastic material contain virtually no acids, water or watervapor, sulphur, or other deleterious substances which are found in manysynthetic plastic materials and which are particularly deterioratingwhen the product is headed as for molding purposes.

A backing layer 5 is preferably made of strong cloth such as cotton ornylon fabric with 64/62 thread, impregmated with a suitable fire andfungus resistant material and joined to sheet 3 with a vinyl adhesive 4.

In another embodiment the plastic sheet 3 may be glued to the metalliclayer 2 by means of a suitable adhesive which is inert with respectthereto. Such an adhesive can comprise, for example, acrylic, vinyl,rubber, cellulose nitrate or cellulose acetate adhesives.

Still another method, preferable in many instances, is to first embossplastic sheet 3, then plate the embossed sheet, then roll-coat theplated embossed sheet.

=If instead of the front coating containing scatter particles, a frontcoating with surface scatter effect is used, the following technique ofapplying such a coating Was found to be practical, replacing the rollcoated front layer described above with reference to FIG. 8. The sheet 3carrying the metal layer 2 is sprayed with a compound available underthe trade designation L/47l from the Union Paste Company of Hyde Park,Mass,

hich compound as mentioned above, consists of 4- parts by weight ofnitrocellulose and 35 parts of the COPOlYf mer known as butylmethacrylate of the type Polycryl 419 made by American Polymer Corp. Thenitrocellulose should be sufficiently nitrated and of low viscosity forlacquer formulation. The solvent used for obtaining a compound suitablefor spraying consists of about equal parts lacquer grade toluol andisopropyl acetate. This spray dries with a surface pattern that has anoptical effect corresponding to that described above with reference toFIG. 6. In order to obtain the size and distribution necessary for apredetermined modification of the field properties of the purely imageforming elements, variations of viscosity, temperature, composition anddrying speed may be necessary, as customary in techniques of this type.

It is also contemplated to first emboss a plastic sheet 3 having a mattesurface to provide the desired scatter effect, then plate the embossedmatte surface, and apply a thin optically essentially clear protectivelayer over the plating.

FIG. 9 is the above mentioned polar coordinate diagram which illustratesthe lateral performance of screens of the present type in comparisonwith conventional screens. In this figure, numeral XI indicates theperformance of a fully specular screen of the above describedconfiguration. XII indicates the performance of a diffusion controlledscreen according to the present invention, XIII that of a conventionalglass beaded screen such as used for purposes of visual education inclass rooms, and XiV that of a so-called Daylight Screen having a thinfiat coating of aluminum on acardboard backing, with a protectiveacrylic spray coating on the front surfaces. The brilliance is plottedin foot-Lambert for the respective angles. The field angle 20 of thescreen Xl is 80. Screen XII provides a field of substantially unimpairedbrilliance over about 60, and a completely usable field for averagepurposes over about 80. Screen XII also provides, beyond its usablefield corresponding to the field of screen XI, an extended field offairly uniformly decreasing brilliance up to approximately 180. It willbe noted that d=40 is the basis of the preceding computations for ascreen according to FIGS. 2 to 5. eyond the predetermined field ofobservation, screen XI has a narrow zone of increased brilliance whichis due to image fusion. Screen XI is wholly unaffected by ambient lightsources outside the field of observation. Screen XIV is more brilliantat the central region, but non-uniform. If screens XI and XII weredesigned for a smaller total field angle or usable field of say 30,their brilliance would be at least equal to that of Screen XIV, andwould be uniform throughout such field.

Generally speaking, screens of types XI and )GI transmit an optimalamount of energy from the projector to the observer, they permitaccurate control of the field of observation, they have optimal colorcharacteristics due to freedom of choice of the reflective metal, theyhave improved resolving power, they have uniform field illumination,they exclude ambient light from the field of observation, and they haveincreased brightness near the field borders which counteracts ambientlight.

Furthermore, experience has shown that screens according to theinvention have imaging and scatter angles which are very favorable withregard to polarization phenomena so that the screens are fullynon-polarizing.

We claim:

I. An optical screen for presenting to a given field of observation theimage of an object region projected on the screen, comprising:

a reflecting surface essentially composed of elementary imaging mirrormeans having optically imaging curvatures arranged in a regular pattern,with the margins of said elementary mirrors contiguously connected suchas substantially to reduce dull light absorbing areas and flatnon-imaging areas, to form elementary imaging mirrors together providinga highly directional projection screen of predetermined optical fieldproperties including uniform distribution over and concentration on thefield of the energy available from the object, and said elementaryimaging mirrors being of dimensions smaller than can be resolved uponobservation by the average viewer at a given viewing distance; and

applied to said reflecting surface an essentially transparent layercontaining embedded therein essentially randomly distributed andessentially separated non-metallic randomly light scattering elements ofdimensions of an order of magnitude slightly larger than the longestwavelength of the visible spectrum including the infrared spectrum, andof an index of refraction different from that of said layer;

whereby said scattering elements contribute to the con trol of saidfield of observation of the reflecting surface as defined by the imagingmirror means.

2. Optical screen according to claim 1 wherein said transparent layer isof essentially uniform thickness.

3. Optical screen according to claim 1 wherein the density anddistribution of said imbedded particles is such that the transparency ofsaid layer is essentially preserved so as not to impair the reflectiveeffect of said reflecting surface.

4. Optical screen according to claim 3 wherein the weight of saidparticles is less than approximately five percent of the weight of thetransparent layer.

5. Optical screen according to claim 4 wherein said layer is essentiallya resinous compound and said particles consist essentially of colloidalsilica.

6. Optical screen according to claim 5 wherein said layer isapproximately five thousands of an inch thick.

7. Optical screen according to claim 5 wherein said colloidal silicaparticles occur in aggregates of an average size of about a thousandmillimicrons.

8. An optical screen for presenting to a field of observation the imageof an object region projected on the screen, comprising: a reflectingsurface essentially composed of parallel strips with undulating bordersthe borders of each strip being symmetric to the axis of said strip, thestrips being contiguous at said undulating borders, each strip havingtransverse curvatures in planes perpendicular to its axis and thesurface of each strip undulating along its axis from concave to convexsuch that said transverse curvatures are larger where the distancesbetween said borders are wider, and smaller where the distances betweensaid borders are narrower, said concave and convex surface having agreater curvature along said axis than the largest transversecurvatures, to form elementary imaging mirrors with a longer and shorteraxis at each widest and narrowest portion respectively of each strip,said elementary mirrors together providing a highly directional opticalscreen of predetermined field properties including uniform energydistribution over the field, and said elementary imaging mirrors beingof dimensions smaller than can be resolved upon observation by theaverage viewer at a given viewing distance; and applied to said surfacean essentially transparent layer of substantially uniform thicknesscontaining embedded therein essentially randomly distributed andeffectively separate non-metallic randomly light scattering elements ofdimensions of an order of magnitude slightly larger than the longestWave length of the visible including infrared spectrum, and of an indexof refraction different from that of said layer; whereby said scatteringelements contribute to the control of the field of the reflectingsurface as defined by said borders and said curvatures.

9. Optical screen according to claim 8, wherein the density anddistribution of said imbedded particles is such that the transparency ofsaid layer is essentially preserved so as not to impair the reflectiveeffect of said refiecting surface.

10. Optical screen according to claim 9 wherein the veight of saidparticles is less than approximately five )ercent of the weight of thetransparent layer.

11. An optical screen for presenting to a given field )f observation theimage of an object region projected )n the screen, comprising:

a reflecting surface essentially composed of elementary imagingv mirrormeans having optically imaging curvatures arranged in a regular pattern,with the margins of said elementary mirrors contiguously connected suchas substantially to reduce dull light absorbing areas and flatnon-imaging areas, to form elementary imaging mirrors together providinga highly directional projection screen of predetermined optical fieldproperties including uniform distribution over and concentration on thefield of the energy available from the object, and said elementaryimaging mirrors being of dimensions smaller than can be resolved uponobservation by the average viewer at a given viewing distance;

applied to said reflecting surface an essentially transparent layer ofessentially uniform thickness; and

embedded in said layer essentially randomly distributed and essentiallyseparated non-metallic randomly light scattering particles of colloidalsilica averaging a grain size of approximately 0.015 to 0.020 miwherebythe transprency of said layer is essentially preserved so as not toimpair the imaging effect of said elementary mirror means and saidparticles contribute to the control of the field of observation of thereflecting surface as defined by the imaging mirror means.

References Cited in the file of this patent UNITED STATES PATENTS2,044,620 Matthai June 16, 1936 2,271,614 Baselt Feb. 3, 1942 2,480,031Kellogg Aug. 23, 1949 2,508,058 Bradley May 16, 1950 2,758,200 FranckAug. 7, 1956 2,804,801 Mihalakis Sept. 3, 1957 UNITED STATES PATENTOFFICE CERTIFICATE OF CORRECTION Patent No, 3,663,339 November 13, 1962Agis I Mihalakis et a1.

It is hereby certified that error appears in the above numbered patentrequiring correction and that the said Letters Patent should read ascorrected below.

In the grant, lines 3 and 4, for "assignors, by mesne assignments, toWilliam J. Snyder," read assignor, by mesne assignments, to William T.Snyder, line 13, for "William J. Snyder, his heirs." read William T.Snyder, his heirs in the heading to the printed specification, line 5,for "assignors, by mesne assignments, to William J. Snyder," readassignors, by mesne assignments, to William T. Snyder, column 1, line67, for "jutaposed read juxtaposed Signed and sealed this 30th day ofApril 1963,,

(SEAL) Attest:

ERNEST W. SWIDER DAVID L. LADD Attesting Officer Commissioner of PatentsUNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No,3,063,339 November 13, 1962 Agis I, Mihalakis et a1,

It is hereby certified that error appears in the above numbered pat entrequiring correction and that the said Letters Patent should read ascorrected below.

In the grant, lines 3 and 4, for "assignors by mesne assignments, toWilliam J. Snyder," read assignor, by mesne assignments, to William T.Snyder, line 13, for "William J. Snyder, his heirs," read William T.Snyder, his heirs in the heading to the printed specification, line 5,for "'assignors, by mesne assignments, to William J, Snyder," readassignors, by mesne assignments, to William T, Snyder, 3 column 1, line67, for "jutaposed" read juxtaposed Signed and sealed this 30th day ofApril 1963 (SEAL) Attest:

ERNEST W. SWIDER DAVID L. LADD Attesting Officer Commissioner of Patents

